The present application relates to a battery using a carbon material as a negative electrode active material.
In recent years, downsizing and weight saving of portable electronic devices represented by a mobile phone, PDA (personal digital assistant) and a laptop personal computer have been actively promoted. As a part thereof, an enhancement in energy density of a battery as a driving power source for such electronic devices, in particular, a secondary battery has been eagerly desired.
As a secondary battery capable of obtaining a high energy density, there are known, for example, secondary batteries using lithium (Li) as an electrode reactant. Above all, a lithium ion secondary battery using a carbon material capable of occluding lithium in a negative electrode and releasing it therefrom is widely put into practical use.
However, in the lithium ion secondary battery using a carbon material for a negative electrode, technologies have already been developed to an extent close to a theoretical capacity thereof. Thus, as a method for further enhancing the energy density, there has been studied a method in which the thickness of an active material layer is increased, thereby increasing a proportion of the active material layer within the battery and decreasing a proportion of each of a collector and a separator (see, for example, JP-A-9-204936).
However, when the thickness of the active material layer is increased without changing a volume of the battery, the area of the collector relatively decreases. Thus, there was involved a problem that a current density to be applied to the negative electrode at the time of charge increases, and diffusion of a lithium ion and electrochemical acceptance of a lithium ion in the negative electrode cannot keep up, whereby metallic lithium is easily deposited. In this way, the metallic lithium deposited in the negative electrode is easily deactivated, resulting in enormous lowering of cycle characteristics. Consequently, it has been difficult to increase the thickness of the active material layer.
Also, when the thickness of the active material layer is increased, or the volumetric density is increased, impregnation properties of an electrolytic solution are deteriorated, and maintenance of the electrolytic solution within an electrode is lowered. Therefore, the current non-uniformly flows within the electrode, whereby cycle characteristics are easily deteriorated. Consequently, it has been difficult to increase the thickness of the active material layer or to increase the volumetric density.
On the other hand, polyvinylidene fluoride (PVDF) is known as a binder. In the case of increasing the thickness of the active material layer, when only polyvinylidene fluoride is used as the binder, there was involved a problem that mobility of a lithium ion is lowered, whereby favorable cycle characteristics are not obtained. Also, when a copolymer which is composed of polyvinylidene fluoride and hexafluoropropylene (HFP) and which is obtained by copolymerizing 100 parts by weight of polyvinylidene fluoride and from about 5 to 12 parts by weight of hexafluoropropylene is used as the binder, there was involved a problem that an electrolytic solution is swollen, and the peel strength of an electrode is lowered, whereby favorable cycle characteristics are not obtained.